Glucose metabolism tests and recurrent pregnancy loss: evidence from a systematic review and meta-analysis

Objective To synthesize the published citations to determine the association between glucose metabolism tests and recurrent pregnancy loss (RPL). Method The electronic databases including PubMed, Scopus and Web of Science were searched for the original articles that evaluated the correlation between glucose metabolism tests including fasting blood glucose (FBG), fasting insulin (FI), homeostatic model assessment for insulin resistance (HOMA-IR), the rate of individuals with HOMA-IR  > 4.5, insulin resistance, fasting glucose/fasting insulin (FG/FI) and FG/FI  > 4.5.and recurrent pregnancy loss with a combination of proper keywords. Results The database search led to finding 390 articles. Detailed screening of titles and abstracts for potential eligibility was performed, and after excluding the duplicated and irrelevant citations, finally, 8 studies were selected to be included in this study, 7 observational studies and one controlled clinical trial. A significant difference in the amount of FI, HOMA-IR, the rate of HOMA-IR  > 4.5, the rate of individuals with insulin resistance, fasting glucose/fasting insulin (FG/FI), and the rate of FG/FI  > 4.5 were found among RPL patients compared to controls. There was no difference when comparing FBG between the groups. Conclusion This study indicates an important link between abnormal glucose metabolism tests and a history of recurrent pregnancy loss. These data may encourage clinicians to request glucose metabolism tests other than FBG in women with recurrent pregnancy loss. Supplementary Information The online version contains supplementary material available at 10.1186/s13098-022-00973-z.


Introduction
Recurrent pregnancy loss (RPL) is defined as the failure of two or more consecutive pregnancies before 20-24 weeks of gestation [1] and is experienced by 2.5% of women trying to conceive [2]. However, it must be noted that the classical definition of RPL includes 3 or more losses [3,4]. The causes may include immunologic and endocrinology disorders, genetic factors, infections, uterine malformation, as well as low quality of the embryo or gametes [5,6]. Although, no risk factors are identified in more than half of the women [7]. It is believed that diabetes or glucose intolerance as an endocrine abnormality could be a possible risk factor for RPL [6,8]. Women with RPL have high fasting insulin and insulin resistance [9].
One of the most critical questions that remain to be answered is if or how glucose metabolism affects pregnancy loss. Hence, we decided to carry out a systematic review and meta-analysis to improve our understanding of the relationship between glucose metabolism tests, including fasting blood glucose (FBG), fasting insulin (FI), homeostatic model assessment for insulin resistance (HOMA-IR), the rate of individuals with HOMA-IR > 4.5, insulin resistance, fasting glucose/ fasting insulin (FG/FI) and FG/FI > 4.5 and RPL. This knowledge might contribute to understand the factors associated to RPL that can be the focus of prevention and management strategies.

Materials and methods
This study was conducted in accordance with the guidance presented in the Cochrane. Handbook for Systematic Reviews of Interventions and reported according to the Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) statement (Additional file 1: Table S1) [10]. This study did not require ethical approval. Our protocol has been prospectively conducted and submitted with PROSPERO, The International Prospective Register of Systematic Reviews.

Study setting
This systematic review was conducted to integrate and improve our understanding of the relationship between glucose metabolism tests including fasting blood glucose (FBG), fasting insulin (FI), homeostatic model assessment for insulin resistance (HOMA-IR), the rate of individuals with HOMA-IR > 4.5, insulin resistance, fasting glucose/fasting insulin (FG/FI) and FG/FI > 4.5 and RPL.

Inclusion and exclusion criteria
Inclusion criteria based on PICOT acronyms were: (1) population: adult women; (2) exposure: patients with a history of two or more pregnancy losses defined as RPL; (3) control: patients with a history of at least one live birth as the control group; (4) outcome: evaluation of different glucose metabolism tests; (5) study design: all original articles which including observational cohorts and case-control studies. Only English articles were assessed. Exclusion criteria were as follows: pregnancy loss due to extrinsic causes, review articles, opinion pieces or guidelines, non-peer-reviewed papers, unpublished reports, and articles in which the date and location of the study were not specified.

Search strategy
The initial search was undertaken in 3

Study registration
The protocol of this study was registered and approved by the ethics review board of Tehran University of Medical Sciences.

Study selection
Titles and abstracts were independently retrieved and reviewed for eligibility by two authors (MG and AS) and non-relevant studies and studies that did not meet the inclusion criteria were excluded. After the initial screening, the remaining full texts were reviewed by SH; articles with not sufficient data and duplicates were identified. Eligibility of included studies and cases reported in case series were double checked at this stage.

Quality assessment, risk of bias
NS and AS independently assessed the quality of included studies using the National Institutes of Health (NHLBI/NIH) quality assessment tools for case-control, cross-sectional studies, and randomized controlled trials [11]. Studies scoring 9 or more were marked as "Good", studies scoring between 7 and 8 were marked as "Fair" and studies rating less than 7 were marked as "Poor".

Data extraction
NS extracted data from included studies to excel.

Outcomes
We investigated 7 potential markers that were used to assess glucose metabolism test in the included studies: (1) fasting blood glucose (FBG); (2) fasting insulin (FI); (3) homeostatic model assessment for insulin resistance (HOMA-IR); (4) the rate of individuals with HOMA-IR > 4.5; (5) the rate of individuals with insulin resistance; (6) fasting glucose/fasting insulin (FG/FI); (7) the rate of individuals with FG/FI > 4.5. The main outcome was to find the association between glucose metabolism tests and RPL.

Data analysis
For outcomes with I 2 > 25% random effect model, and for outcomes with I 2 < 25% fixed effect model was used for data pooling. For continuous outcomes, Standard mean difference (SMD) was used for reporting combined results. SMD is more generalizable than regular mean difference even when using similar units [12]. For dichotomous outcomes, odds ratio was the selected measure for summarization. Leave-on-out sensitivity analysis was done to detect an effect of individual studies on the overall results. Data analysis was done with RevMan 5.4. Data presented in graphs were extracted using Plot Digitizer software.

General outcomes
The database search led to finding 390 articles. Detailed screening of titles and abstracts for potential eligibility was performed, 248 were duplicated, and the irrelevant papers were removed. According to inclusion and exclusion criteria, the full texts of 22 articles were reviewed. Three studies did not have a control group and were excluded. Two other prospective cohort studies considered diabetes and insulin resistance as exposures and RPL as an outcome, so we excluded these studies because they were not comparing patients with a history of RPL to patients without a history of RPL [13,14]. Finally, 9 studies [8,9,[15][16][17][18][19][20][21] were selected to be included in this study. Figure 1 presents the PRISMA flow chart of the study selection and screening process.

Records identified from:
Databases ( Seven studies reported combinable outcomes and were used for quantitative synthesis and meta-analysis. Study by Habets et al. [17] reported some combinable outcomes but they chose median and interquartile range rather than mean and standard deviation as their measure of central tendency so we could not pool their findings with other studies, and only report them separately. Study by Zolghadri et al. [19] was the only study reporting glucose tolerance test (GTT) as an outcome and did not report any combinable outcome so this study was not included in meta-analysis.

Risk of bias
Two studies were labeled as "Good" and seven others were labeled as "Fair". Detailed results of quality assessment for each study have been summarized in Additional file 1: Table S1.
A subgroup analysis based on the inclusion of pregnant women was done. In both subgroups, FBG was higher in RPL group (not significant). Pooled estimated for subgroup of studies that included pregnant women and subgroup of studies that did not include pregnant women are the following respectively: SMD = 0.56, CI 95% = [− 0.71-1.84], P = 0.39 and SMD = 0.21 CI 95% = [− 0.28-0.71], P = 0.40. Inclusion or exclusion of pregnant women does not seem to have a significant effect (Test for between-subgroup differences P = 0.62) (Additional file 1: Fig. S1).
Another subgroup analysis based on definition of RPL was done. Interestingly in RPL ≥ 3 subgroup FBG was significantly higher among RPL patients (SMD = 0.31, CI 95% = [0.00-0.61]. However, this difference between the subgroups does not seem to have a significant (P = 0.97) (Additional file 1: Fig. S2).
Habets et al. [17] study result was consistent with our findings, this study compared fasting blood glucose between three groups, patients who had 2 or three pregnancy losses, patients who had more than 4 pregnancy losses and control group, the results were the follow-

Glucose tolerance test (GTT)
Zolghadri et al. [19] in a randomized clinical trial compared glucose tollerance test (GTT) between non pregnant patients with history of RPL(n = 133) and control(n = 70) group. Patients with blood glucose levels between 140 and 200 mg/dl after ingestion of 75 g glucose were classified as impaired GTT. 17.6% of patients with RPL history had impaired GTT, 5.4% of control group had impaired GTT, OR = 1.34, CI 95% = 0.25-2.42, P = 0.17.

Sensitivity analysis
We performed a sensitivity analysis by excluding the results of each study to assess how their findings have effect on the overall results. Table 2 presents the results of leave-one-out sensitivity analysis.

Discussion
The present study, to our knowledge, was the first systematic review and meta-analysis to assess the relationship between recurrent pregnancy loss (RPL) and glucose intolerance. We found a significant difference in the amount of FI, HOMA-IR, the rate of HOMA-IR > 4.5, the rate of individuals with insulin resistance, FG/FI, and the rate of FG/FI > 4.5 among RPL patients compared to controls. There was no difference when comparing FBG between the groups. Several factors have been linked to the etiology of RPL. However, the incidence of unexplained RPL is about 40% in most studies [9,15,23]. PCOS is the most reported endocrine disorder in women, and it is claimed that insulin resistance and hyperinsulinemia are one of the main causes of the pathophysiology of RPL [24].
In the 9 studies included in this systematic review, 7 studies with 889 individuals reported the FBG in both patients with RPL and controls. We found a non-significant association between FBG and recurrent miscarriages. The majority of the reports on this topic are in line with our findings [8,15,17,18]. Recently, a study by Habets et al. aimed to evaluate vascular and metabolic status of women with RPL and women with a history of uncomplicated pregnancy [17]. They found that there was no significant difference between FBG and RPL.
In contrast to these findings, Edugbe et al. found that women with RPL had a higher mean FBG [16].
Two studies by Crane et al. [14] and McCoormack [13] et al. were excluded from our study but their results are helpful. Crane et al. compared pregnancy loss between diabetic and non-diabetic patients; they reported no significant difference between these two groups. And McCormack et al. reported a high rate of hyperinsulinemia among patients with recurrent pregnancy loss.
Our findings suggest that there is a significant association between recurrent miscarriages and IR in patients with RPL by calculating the pooled SMD of HOMA-IR as a potential biomarker for assessing IR in patients. This finding was further confirmed by analyzing the odds of HOMA-IR > 4.5 between the RPL and comparing group.
Consistent with our results, Ispasoiu [9] and Tian et al. [25] found that in patients with RPL, the mean level of HOMA-IR was significantly higher compared to normal subjects, although both of these studies evaluated a small sample size. These contradictory findings were explained by Wang et al. [18]. Different approaches were used in their investigation to examine how the IR status between the case and control groups differed. Between the 5th and 13th weeks of pregnancy, they performed an oral glucose tolerance test (OGTT) and an insulin-releasing test. They found that only a short time after consuming the oral glucose solution did the two groups significantly differ from one another (Based on 1-, 2-, and 3-h of both glucose and insulin tests). Furthermore, they stated that no significant differences between the levels of FBG, FI, and HOMA-IR were observed when comparing two groups, indicating that relying solely on these indicators to detect the presence of IR in RPL-affected women is inadequate. The findings of our meta-analysis suggested that patients with RPL have significantly higher mean fasting insulin (FI) levels than controls. These results suggest that hyperinsulinemia may be present in women who experience recurrent miscarriages. Hyperinsulinemia is associated with decreased expression levels of glycodelin and insulin-like growth factor binding protein 1 (IGFBP-1), which are responsible for the immune response of the mother's immune system towards the implanted embryo and facilitation of the adhesion process of blastocysts, respectively [9,26,27]. It is also suggested that hyperinsulinemia could increase the level of plasminogen activator inhibitor-1 and induce villous thrombosis then causing trophoblastic hypoplasia and miscarriage.
Although the gold standard for identifying IR is the clamp technique [28], this test is not widely used by clinicians because of its time-consuming procedure and complex nature [15]. Therefore, most of the studies used the fasting glucose to fasting insulin (FG/FI) ratio and the HOMA-IR. In our study, we identified a significant increase in both the mean of FG/FI and the odds of FG/ FI > 4.5 among women with RPL.

Clinical implication
Although there is some debate about the exact biomarker alterations that occur in women who have had a history of RPL, the majority of studies agree that these women experience increased IR. The screening of pregnant mothers with a history of RPL in terms of metabolic status and blood sugar level should be prioritized to investigate possible IR states. FI, HOMA-IR, and FG/FI as IR indicators in these individuals need to be assessed in light of our findings.

Strengths and limitations
Our study has several strengths. We performed a comprehensive systematic review of three databases in line with the PRISMA statement with no date and language restrictions. To adjust serum levels of different markers for assessing IR, pooled effect sizes of relevant outcomes were calculated using the inverse variance method.
Despite these strengths, we also faced number of limitations. The most important one lies in the fact that most of the included studies were observational, leading to the fact that these studies are limited-designed in nature and carry inherent biases. The majority of the studies included in the meta-analysis were of fair quality, which could result in weakening the strength of this meta-analysis. The paucity of studies in this field may contribute to publication bias.
In conclusion, our study revealed a significant association between abnormal glucose metabolism tests and a history of recurrent pregnancy loss. These findings may persuade medical professionals to order further glucose metabolism tests in women who experience repeated miscarriages in addition to FBG.